Photosystem II (PSII) is a multisubunit complex that utilizes light energy to drive the upstream electron transport reactions of photosynthesis. Within PSII these reactions ultimately lead to the reduction of membrane diffusable ubiquinone and the oxidation of water. The core complex consists of a number of membrane proteins which are called (molecular weight given in parentheses) D1 (32 K), D2 (34 K), Cp-47 (47 K), Cp-43 (43 K) and cyt b559 (composed of a 4K subunit and a 9 K subunit), and several extrinsic proteins which are usually referred to by their molecular weights of 33K, 23K and 17K. Within a single PSII core, two copies of the latter four components are present and the remaining components are present as single copies. In PSII membranes, the light absorbing chlorophyll antennae are bound by light harvesting chlorophyll a/b protein (LHCP) with molecular weights of 25K or 27K and are present at an estimated 15-20 per reaction core. Differential scanning calorimetry (DSC) has been performed on PSII preparations (Thompson et al., 1986 Biochem 25: 6161) and on PSII core preparations (Thompson et al., 1989 Biochem 28: 6686). The authors reported that deconvolution analysis consistently gave 5 thermal transition peaks (labeled A1, A2, B, C and D) for preparations scanned at a fixed concentration of Triton X-100 (0.01%). Furthermore, they reported that the A2 transition peak disappeared when the 23K and 17K extrinsic proteins were removed and under conditions when the complex was oxidized. Based on these experiments, these investigators proposed that the extrinsic proteins limit the solvent accessibility of an oxidizable element (more than likely cyt b559). This hypothesis can be confirmed by the proposed experiment. PSII core preparations lacking the 23K and 17K extrinsic proteins can be modified to include ascorbate (a mild reductant ). If indeed the A2 transition is affected by the oxidation state of the core complex, then the inclusion of a mild reductant in the final buffer should prevent the dissappearance of the peak.